Myopia and hyperopia are refractive errors that may occur in one or both eyes. Myopia or nearsightedness occurs when light rays entering the eye focus in front of the retina rather than directly on it, as in the normal eye. Myopia often results when the curvature of the cornea is too steep. In contrast, hyperopia or farsightedness occurs when light rays entering the eye focus behind, instead of directly on, the retina. With hyperopia, the curvature of the cornea is too flat. Ordinarily, the error in the focal point of light entering the eye is corrected with glasses or contact lenses so that a clear, sharp image is formed on the retina and transmitted to the brain.
Over the years, eye surgeons have developed various surgical procedures to correct these refractive errors. One of the most common techniques is radial keratotomy (RK), a surgical procedure used to correct myopia; it is not used to correct hyperopia. During RK, a surgeon makes a series of single-cut incisions in the surface of the cornea in order to flatten it. The change in the curvature of the cornea moves the point at which light focuses from a position forward of the retina to a position directly on the retina.
Automated lamellar keratoplasty (ALK) is another technique utilized to correct refractive errors in the eye. An essential step in this procedure is slicing a portion off of the top layer of the cornea. This technique is used to correct hyperopia, higher degrees of myopia and presbyopia, a condition where the lens inside the eye loses its ability to focus on near objects.
During ALK, an automated device containing an oscillating blade cuts across the central cornea over the pupil, leaving the deeper layers of the cornea uncut. A thin disc of corneal tissue is removed. Intraocular pressure inside the eye pushes the uncut corneal layers outward, increasing the curvature of the cornea. Then the removed disc of corneal tissue is replaced. These steps correct the hyperopic eye. In the myopic eye, two discs of tissue may be removed from the cornea and the first disc replaced, to decrease the original curvature of the cornea.
A third corrective procedure known in the art is hexagonal keratotomy (HK). This technique involves making a hexagonally-shaped incision surrounding the pupil, effectively creating an "island" around the pupil. Pressure inside the eye forces the "island" outward, increasing the curvature of the cornea. This procedure is used to decrease hyperopia and to increase myopia.
A fourth corrective procedure known in the art is Photorefractive Keratectomy Laser surgery (PKL). With this method, a laser vaporizes portions of the cornea to flatten it in order to correct myopia. The PKL technique is also used to correct hyperopia by burning off more peripheral cornea tissue in order to recontour the corneal surface.
Another corrective procedure known in the art is Astigmatic Keratotomy (AK). This procedure is essentially a subtype of RK wherein transverse and radial incisions are made in the cornea.
Another corrective procedure known in the art involves using implants to surgically correct refractive errors. For example, U.S. Pat. No. 4,781,187 to Herrick teaches a method and implant to correct hyperopia by inserting a triangular implant into an incision made in the cornea. One may correct other refractive errors such as myopia using this method by inserting an implant having a different shape.
Although the above-described techniques have been widely used in conjunction with the correction of refractive errors, they are not entirely satisfactory. For example, although it usually corrects myopia, RK sometimes produces certain undesirable side effects, including overcorrection. In other words, the adjustment made by the RK procedure causes the cornea to flatten too much, with the result that the focal point of the eye lies behind the retina, producing hyperopia. This is effectively the condition that naturally exists with farsighted individuals.
The ALK procedure is very unpredictable in its results and may also result in overcorrection. ALK is a more complicated surgical procedure than RK and has the inherent disadvantage of cutting the cornea directly over the pupil, through which the light of vision passes. In contrast, RK employs radial incisions which do not extend across the pupil. In addition, the ALK and HK procedures are not feasible methods to repair overcorrection in eyes that have previously undergone an RK procedure.
Single-blade instruments for use in RK procedures are well known in the art. For example, U.S. Pat. No. 5,376,099 to KMI, Inc. teaches an instrument wherein the blade includes a projecting, short cutting edge and a recessed blunt edge. This instrument is not designed to handle the reversal of overcorrected RKs or naturally occurring hyperopia.
Thus, there is a need for a method and instrument for reversing an overcorrected RK surgery and for correcting a naturally occurring hyperopic eye that does not include the above-described disadvantages.